Gunshot Residue Evidence and the Trayvon Martin Case

My presentation might have exhausted the class’s interest in gunshot reside analysis, but for anyone still interested, some of the recently-released forensic evidence in the Trayvon Martin case reflects the principles we discussed, without delving into the detailed studies. Investigators found one GSR particle on the sleeve of George Zimmerman, the shooter, and two particles on Trayvon Martin’s sweatshirt.

This, of course, should mean very little in light of the frequent false positives we discussed. Regardless of whether Martin and Zimmerman actually tussled, merely being in close quarters was enough to transfer particles between the two. And even a scrupulous lab technician could get a false-positive result based on contamination from the activities that followed the shooting, including paramedics responding and hurriedly rendering aid at the scene. In short, this evidence should mean very little.

Instead, outside experts retained by the press have suggested the GSR results necessarily mean Martin died from a “contact shot,” meaning the barrel was physically pressed against his body when it fired. This would ostensibly support Zimmerman’s defense, that he only fired once the two were fighting in close-contact. Though the defense attorneys have not said whether they intend to rely on the GSR evidence to support their self-defense theory, this case could become a high-profile case study in the shortcomings of this pervasive evidence. But even if the GSR proves crucial, the sensationalized aspects of this case are an apparently poor context for any thoughtful progress toward better understanding the science.

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